It has been suggested to convert normally insulating thermoset and thermoplastic polymers or prepolymers into electrically conducting compositions by admixing therewith various electrically conductive metal particles, such as silver and copper. The possible applications for such conducting compositions are quite extensive. For instance, it would be desirable to use these conducting compositions for electrostatic discharge protection, electromagnetic interference shielding, as adhesives for interconnect technology as replacements for solder joint connections.
However, there are problems associated with this technology. Often, high levels of the metal particles are needed to achieve the desired level of electrical conductivity, especially in high current carrying applications, such as interconnect technology, where the higher levels of electrical conductivity must be achieved. A loading level of 50% and higher is often required for such applications. A second serious problem with these metal fillers is that the more useful ones, silver and copper, tend to corrode in a variety of ambients. The metal has a tendency to oxidize, and thus, an oxidized layer of copper or silver will be on the surface of the particles, and will result in loss of conductivity, or at least a decrease in the conductivity.
Although both copper and silver are called "noble," they readily corrode in a variety of ambients. In oxygen saturated deionized water, copper and silver dissolve at a rate of about 0.2 .mu.m/day, with no evidence of passivation. Both metals are susceptible to the presence of pollutants, notably chlorides and sulphur.
In the presence of humidity, pollutants and an applied electrical field (not an uncommon situation for the electronic parts in operation), copper and silver dissolve from the more positive metallic part and plate at the more negative part as dendrites. The formation of dendrites can result in electrical shorts that can lead to the failure of the electronic device.
Furthermore, with increasing line density and decreasing dimensions, ion accumulation alone, without dendrites, can ruin the designed electrical performance of the product. Use of inhibitors, such as benzotriazole (BTA), greatly improves the corrosion resistance of copper and silver. However, BTA is not very protective against dissolution at potentials above the open circuit potential, as it may occur in a galvanic contact with gold or platinum, or in service, with the applied field.
Moreover, to circumvent these problems, copper and silver metal particles are often coated with other more stable metals, such as gold or nickel. This is done by electroless or electroplating processes, which add additional processing costs and pose potential environmental concerns, due to the chemical make-up of the electroplating baths.